The present invention relates to a novel process for producing 4-hydroxy-2,4,6-trimethylcyclohexa-2,5-diene-1-one, and more particularly, to a process for preparing 4-hydroxy-2,4,6-trimethylcyclohexa-2,5-diene-1-one from 2,4,6-trimethylphenol, and hypohalogenous acid or salt thereof.
4-Hydroxy-2,4,6-trimethylcyclohexa-2,5-diene-1-one (hereinafter referred to as "HTCD") is a compound represented by the following formula: ##STR1##
It has long been known that HTCD is converted into 2,3,5-trimethylhydroquinone on heating in the presence of alkalis. This 2,3,5-trimethylhydroquinone is a compound which has received increasing attention in recent years as a starting material for use in the preparation of vitamin E. It has thus bendesired to develop a method of preparing HTCD inexpensively.
Several methods are known for the synthesis of HTCD. Typical examples are:
(1) a method in which 2,4,6-trimethylphenol (hereinafter referred to as "TMP") is reacted with monopotassium persulfate (see Bamberger, Ber., 36, 2033); PA1 (2) a method of oxidizing TMP with periodic acid; (see E. Adler et al., Acta. Chem. Scand., 29, 909 (1975)) PA1 (3) a method of electrolytic oxidation of TMP (see C. G. Beddows & D. V. Wilson, J. C. S. (Perkin 1), 2337 (1973)); and PA1 (4) a method of oxidizing TMP with molecular oxygen (see DT-OS 2,747,497 and Japanese patent application Laid-Open Nos. 121,252/1975, 127,937/1974).
These methods, however, are not desirable from an industrial or commercial standpoint. In the methods (1) and (2), specific and expensive oxidizing agents are used, and therefore they are disadvantageous from an economic standpoint. The method (3) is disadvantageous for its industrial practice because it needs a very specific reactor.
Of the above-described methods, the method (4) is considered to be an industrially most promising method.
For example, DT-OS 2,747,497 discloses a method in which TMP is reacted in the presence of isopropyl alcohol under an air pressure of 100 atmospheres by the use of a catalyst containing cobalt to prepare HTCD in the yield of 99%. In the commercial practice of this method, however, the danger of explosion is very large, because the contacting of high-pressure air with organic compounds often causes an explosion.
Japanese patent application Laid-Open No. 121252/1975 describes that when a solution of TMP dissolved in NaOH-containing water is repeatedly passed through a multi-stage reaction column charged with pure oxygen gas under a pressure of 70 kg/cm.sup.2 G by means of a pump, there is formed HTCD at the selectivity of 70%. In accordance with this method, there is no fear of explosion. However, in such a high-pressure oxygen gas atmosphere, substances are liable to become very inflammable, and there is a danger of even metals being set on fire. It is therefore very dangerous to allow a large amount of oxygen gas to stay in the reaction column. Thus, on the whole, it is not considered that the danger associated with the industrial practice is reduced.
In general, production costs of high-pressure reactors are high, obviously constituting a factor in increasing the costs of reaction products.
Japanese patent application Laid-Open No. 127937/1974 discloses a method of oxidizing TMP with oxygen under atmospheric pressure in the presence of a tetraphenylporphyrin cobalt complex catalyst. This method, however, is not industrially advantageous because the complex catalyst is very expensive.
As described above, the conventional methods suffer from industrial disadvantages and are not always sufficiently satisfactory.